GPU Tessellation

Damon Rocco
Tessellation: The filling of a plane with
polygons such that there is no overlap or gap.
In computer graphics objects are rendered as
a triangular mesh
◦ A triangle is always a plane
Thus, the triangles in a mesh can be
tessellated to increase detail
In software tessellation provides an
interesting way of enhancing detail
In hardware tessellation allows a simple mesh
to be sent down to the GPU, converted to a
complex mesh, and then displayed
◦ Decrease memory to the card
◦ Increase rendering performance by decreasing the
number of polygons through the full pipeline
With programmable tessellation what objects
get tessellated can be decided by the
This allows for objects closer to the screen to
be tessellated while objects far away are not
◦ Detail actually increases as objects get closer
instead of just getting bigger
◦ Resources aren’t wasted on meshes that are too far
away for tessellation effects to be viewed
Xbox 360: ATI Xenos graphics chipset
◦ The first geometry tessellation unit
◦ “Programmable”
ATI Radeon HD 5xxx series
◦ The first DX11 card
NVIDIA “Fermi”
◦ NVIDIA’s first DX11 card
DirectX 9: Dynamic Terrain Rendering on GPU
Using Real-Time Tessellation. Natalya
Tatarchuk. ATI Research published in
DirectX 10: Instanced Tessellation in
DirectX10. Andrei Tatarinov. GDC ‘08
DirectX 11: New pipeline features three
tessellation stages, 2 are programmable
Natalya Tatarchuk
Goal: Create a software library that would
allow tessellation exclusively on ATI cards.
Product: ATI GPU Tessellation library. An ATI
only library that worked in conjunction with
DirectX 9 (also 10.x) and was capable of
tessellating meshes
A flat 900 polygon grid becomes a 1,000,000
polygon mountain-scape at inter-actable
Andrei Tatarinov
In the past complex shading models were
used to hide lack of detail in a polygonal
They can only do so much…
The solution is to use on-the-card
tessellation to increase the physical detail in
the meshes
A highly detailed mesh can be sent to the card
as a simple mesh and a displacement map
Trades ALU operations for memory bandwidth
ALUs scale faster than bandwidth
Tessellation can also be used by Animators to
make their job easier
◦ Animate a low polygon mesh
◦ Tessellate and get detail for free
New “patch” primitive defined by a set of
control points
Operation called refinement generates
triangle from each patch
Per-patch operations
◦ Level of Detail computation
◦ Transfer of Basis
 Bezier -> B-spline
 B-spline -> NURBS
 Etc.
Generating topology
◦ Generates a set of (u,v)-points
in the tessellation domain
Programmable hardware tessellation
◦ 3 stages
 2 programmable shaders
 1 fixed function configurable tessellator
◦ New primitive “patch”
 Curved surface
 Easily converted to triangles
Programmable tessellation can be imitated
using DirectX 10 features:
◦ Geometry Shader and Instancing 2.0
Geometry Shader cannot do tessellation itself
◦ Outputs triangles serially
◦ Maximum output size of 1024 scalars
 16x16 grid of float4s
Instead we can save small pre-tessellated
patches as instances
The pre-tessellated patches represent the
results of tessellating every input patch
This data is combined in the vertex shader
to produce the desired effects
Vertex shader inputs are too small to handle
an entire mesh
◦ Must be bound to shared buffers
The tessellation mesh instances are of
varying quality, a LoD factor is used to
determine which mesh to select
Tessellation factors can change across a
mesh as each patch edge has its own
Per-patch tessellation factors requires
multiple draw calls (it won’t in DX11)
Since all of the meshes are stored in vertex
buffers the only inputs are the primitive id
and the vertex id
U = Vertex ID mod LoD
V = Vertex ID div LoD
LoD is based on vertex position
Mesh: 6118 patches, 256 vertices each, 8800 GT
Tessellation presents a means of significantly
increasing detail without a performance cost
Tessellation is possible with DirectX 10
Some of the hiccups with the DirectX 10
implementation will be fixed by future
hardware implementations
Kevin Gee
D3D11 HW Feature
◦ Required for DirectX 11 compatibility
D3D11 Only
◦ No direct backwards compatibility
Fundamental primitive is “patch”
◦ Not triangles
Superset of Xbox 360 tessellation
3 new stages added for Tessellation
Takes patch control points as an input
2 Output paths:
◦ Output of basis converted
control points to Domain Shader
◦ Output of the control points, a
tessellation factor, and
tessellation modes to the
Tessellator operates on a per patch basis
Again 2 output paths:
◦ Outputs U,V points to Domain Shader for further
◦ Outputs topology for primitive assembly
Either receives a set of points from the Hull
Shader or the Tessellator.
The Domain Shader is invoked once per point
It outputs verticies
Images courtesy of ATI
Creates a new authoring pipeline
◦ 1-pass process from input to optimally tessellated mesh
◦ Makes both animation and real-time applications faster
◦ Allows for a higher level of detail
Images courtesy of NVIDIA Gamefest 2008
Ignacio Castano
Tessellation lets us send down low polygon
meshes to save memory and bandwidth which
are the main bottlenecks
Tessellation allows for view dependent levels
of detail so the GPU doesn’t waste time
rendering unseen triangles.
Subdivision Surfaces are a well explored
technique for increasing the detail of a mesh.
Previous GPU implementations required
multiple GPU passes
The new DirectX 11 tessellation hardware
allows us to do subdivision surfaces in a
single pass
The hull shader is used for control point
Input is a face and its neighboorhood
Output is a regular bicubic bezier patch of
the face
The Domain Shader evaluates the bicubic
Bezier patches and corresponding tangents
Reorders face patches for consistent
Requires 69 instructions
TATARCHUK N.: Dynamic Terrain Rendering on
GPU Using Real-Time Tessellation. ShaderX7
(Dec. 2008).
Tatarinov, A.: Instanced Tessellation in
DirectX10. GDC 2008. February 2008.
Gee, K.: Introduction to the Direct3D 11
Graphics Pipeline. Nvision 2008.
Castano, I.: Tessellation of Displaced
Subdivision Surfaces in DX11. Gamefest 2008

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